1. Field of the Invention
The present invention relates to a gas filled switching electric discharge tube. More particularly, the present invention relates to the structure of a gas filled switching electric discharge tube in which the voltage characteristic at the time of electric discharge is improved.
2. Related Art
The gas filled switching electric discharge tube includes: a cylindrical body made of insulating material such as ceramic; and a first electrode and a second electrode for airtightly closing both ends of the cylindrical body, wherein an electric discharge gap is formed between the first electrode face of the first electrode and the second electrode face of the second electrode, and gas is filed into an airtightly closed space which is formed in the cylindrical body including the electric discharge gap. Due to the above structure, electric discharge is generated between the first electrode face and the second electrode face.
In the case where switching is conducted in the thus composed conventional switching electric discharge tube after it has been left in a completely dark place, the electric discharge voltage (FVs) of the first time is necessarily higher than the electric discharge voltage (Vs) of the second time and the times after that. The reason why is that, as the switching electric discharge tube has been left in a dark place, it is impossible for photo-electrons, which always excite the filled gas in a bright state, to provide an excitation effect (the photo-electron effect).
Conventionally, the life of electric discharge of the electric discharge tube has been extended and the increase of the FVs characteristic, in a life test, has been prevented by arranging carbon trigger wires on an inner wall face of the cylindrical body made of ceramic and devising various methods of arrangement.
For example, in order to improve the voltage characteristic of this type of switching electric discharge tube in the case of discharging, the following arrangements have been proposed. Metallized faces are formed on both end faces, which come into contact with the electrodes, of the cylindrical body made of ceramic, and trigger wires are provided which come into contact with the metallized faces and extend on an inner wall face of the cylindrical body, or alternatively trigger wires are provided which do not come into contact with the metallized faces but extend on the inner wall face of the cylindrical body.
In the conventional switching electric discharge tube of this type, as the electrode and the cylindrical body made of ceramic are joined to each other by means of soldering, the electrode is made of a low thermal expansion alloy, the coefficient of thermal expansion of which is close to that of a ceramic, such as covar or iron-nickel alloy. However, as the electric conductivity of the above material is low, generation of the creeping corona discharge in a dark place is delayed, which raises the switching electric discharge starting voltage FVs of the first electric discharge to higher than that of the second electric discharge.
In order to solve the above problems, the following countermeasures have been taken conventionally. According to Japanese Unexamined Patent Publication No. 63-24576, the electrode itself is made of copper, and hydrogen gas is partially filled into the electric discharge tube. According to Japanese Unexamined Patent Publication Nos. 3-77292 and 3-77293, the electrode made of covar or iron-nickel alloy is plated with copper or a copper alloy.
However, in the conventional gas filled switching electric discharge tube described above, when only carbon trigger wires are formed in the cylindrical body or when only an arrangement of the carbon trigger wires is devised or when only the electrode itself is made of copper or only when the electrode made of covar or iron-nickel alloy is plated with copper or copper alloy, it is difficult for the extension of the life of electric discharge to be compatible with the prevention of a rise in the FVs characteristic in the case of a life test. In view of the above problems caused in the prior art, the present invention has been accomplished.
Accordingly, an object of the present invention is to provide a gas filled switching electric discharge tube, the characteristic of which will be described as follows. The low electric conductivity, which is a property of the material composing the electrode itself, is improved by conducting plating on the electrode. Further, when an interval of the electric discharge gap and an interval between the electrode face and the trigger wires are regulated, the extension of the life of electric discharge and the prevention of a rise in the FVs characteristic in the case of a life test can be accomplished.
According to the present invention, there is provided a gas filled switching electric discharge tube comprising: a cylindrical body made of insulating material; a first and a second electrode for airtightly closing both ends of the cylindrical body so that an electric discharge gap is formed between a first electrode face of the first electrode and a second electrode face of the second electrode, and an airtightly closed space formed in the cylindrical body being filled with gas; metallized faces formed on both faces of the cylindrical body, the first and the second electrode being joined to the cylindrical body on both the end faces of the cylindrical body; first trigger wires formed on an inner wall face of the cylindrical body, connected with the metallized faces; second trigger wires formed on the inner wall face of the cylindrical body, not connected with the metallized faces; at least one of the first electrode face of the first electrode and the second electrode face of the second electrode is plated with copper or silver; and an interval of the electric discharge gap being made to be larger than a distance from the second trigger wires to the first or the second electrode face.
The cylindrical body is a cylinder, the first and the second electrode face are substantially circular and formed around the central axis of the cylindrical body, the first and the second electrode face are arranged being symmetrically opposed to each other, the first trigger wires extend from the metallized faces in the axial direction on the inner wall face of the cylindrical body, however, the first trigger wires do not reach a central portion of the cylindrical body, the second trigger wires extend in the central portion of the cylindrical body in the axial direction, and a distance (d) from the second trigger wires to the first or second electrode face is a radial distance from an outer surface of these electrodes to an inner wall of the cylindrical body.
The electric discharge gap (t) is a distance between tips of the first electrode face and the second electrode face facing to each other.
At least one of the first electrode face of the first electrode and the second electrode face of the second electrode is plated with copper or silver so that a thickness of the plated layer is 10-20 xcexcm.
The number of the second trigger wires is larger than the number of the second trigger wires.
The first trigger wires extend from the metallized face in the axial direction along an inner wall face of the cylindrical body, however, they do not extend over a central area, and, on the other hand, the second trigger wires extend in the axial direction at the central area.
The first trigger wires include a pair thereof spaced by 180xc2x0, one extending in the axial direction from one of the metallized faces and the other extending in the axial direction from the other of the metallized faces.
The pair of the first trigger wires are respectively composed of a plurality of trigger lines arranged close and parallel to each other.
The length of the first trigger wire in the axial direction is not more than ⅓ of the length of the cylindrical body in the axial direction.
A plurality of the second trigger wires are arranged at substantially regular intervals between a pair of the first trigger wires which are arranged at an interval of 180xc2x0.
The length of the second trigger wire in the axial direction is not less than xc2xd of the length of the cylindrical body in the axial direction.
A plurality of recessed portions are provided on at least one of the first and the second electrode faces.
The recess portions are hemispherical recess portions.
The plurality of recess portions are uniformly arranged at regular pitches of 0.1-1.0 mm.
The first and the second electrode faces are arranged to be symmetrically opposed to each other, central portions of the electrode faces are hollowed with respect to the peripheral portion, and the plurality of recess portions are formed in the hollow portion.
The cylindrical body is made of ceramic, and the first and the second electrode are made of iron-nickel alloy such as 42 alloy or iron-nickel-cobalt alloy such as covar.
The first and the second electrode are joined to the cylindrical body by means of soldering.